Add -lrapidcheck to pkg-config module

This adds the library to rapidcheck.pc, so that it doesn't have
to be specified manually in projects that consume it.

The other modules don't need it because they have rapidcheck in
their Requires field.

CMakeLists.txt

@@ -128,7 +128,7 @@ set(PKG_CONFIG_DESCRIPTION_SUMMARY "C++ framework for property based testing ins
 set(PKG_CONFIG_VERSION)
 set(PKG_CONFIG_LIBDIR "\${prefix}/lib")
 set(PKG_CONFIG_INCLUDEDIR "\${prefix}/include")
-set(PKG_CONFIG_LIBS)
+set(PKG_CONFIG_LIBS "-lrapidcheck")
 set(PKG_CONFIG_CFLAGS "-I\${includedir}")
 
 configure_file(